The present invention is directed to a material for sealing a ceramic body, a lamp using this material as a frit seal, and a method for sealing a ceramic body, such as a high intensity discharge lamp, with this material.
FIG. 1 shows a discharge vessel 1 for a conventional ceramic metal halide, high intensity discharge lamp. The discharge vessel 1 includes a ceramic arc tube 6 filled with a chemical fill 8 and into which electrodes 14 are fed through ceramic capillaries 2. The arc tube has halves 17a,b that are joined at seam 5, where the electrode ends 3 are inside the respective halves. The distal ends of the capillaries are each sealed with a respective frit seal 9. The capillaries 2 are long relative to the size of the arc tube 6 (e.g., each capillary is longer than the diameter of the arc tube) in order to reduce the temperature of the frit seal 9. Long capillaries are required because the conventional material of the frit seal 9 is reactive with the chemical fill (e.g., a mercury and a mixture of metal halides) in the arc tube at elevated temperatures. If the frit seal is too close to the chemical fill, the heat-induced reaction of the frit seal material with the chemical fill can cause a color shift and reduce lamp life.
As described in U.S. Pat. No. 4,076,991, a material conventionally used to seal an inlead to a ceramic body is made from dysprosia (Dy2O3), alumina (Al2O3), and silica (SiO2). The weight percentages (wt %) of the material (without the binder) may vary in the ranges of 20-80 wt % dysprosia, 0-55 wt % alumina, and 5-45 wt % silica. The Patent indicates that outside these composition limits, the seals “can be made only with difficulty and with large mechanical tension in the sealing joints upon cooling.” In practice, the material of a typical frit seal is approximately 55 wt % dysprosia, 20 wt % alumina, and 25 wt % silica, which are well inside these composition limits.
International Patent Publication WO 2008/020406 describes a method for making the frit seal in which the material of the frit seal is pressed and sintered into the form a ring that is mounted on the extended electrodes and melted to seal the ceramic discharge vessel. This Publication further describes a material for the frit seal that includes 25-60 wt % cerium oxide (Ce2O3), 12-64 wt % alumina, and 3-50 wt % silica, and more preferably 30-57 wt % cerium oxide, 20-48 wt % alumina, and 10-22 wt % silica. Using cerium oxide lowers the frit melting point, thereby facilitating manufacture of the discharge vessel.